Generally speaking, a capsule filling machine for the production of pharmaceutical capsules basically comprises a drum which rotates about a vertical axis of rotation, and a circular fixed tank which holds the pharmaceutical material to be fed into the capsules by dosing.
The drum handles and positions the capsules to be filled with the pharmaceutical material by separating the capsule lid from the capsule body then closing them again once filled. The drum, to which the tank containing the material to be dosed is connected, also has a plurality of doser elements designed to pick up relative doses of material from the tank and, respectively, to deposit each dose in the capsule body before the capsule body is closed again with the relative capsule lid.
According to a known dosing method the doser elements, each consisting of a hollow punch, forming a hollow cylinder and housing a piston moving with alternate motion, perform the following operating steps one after another: a vertical stroke by the hollow cylinder into the tank, for immersion in the pharmaceutical material until the cylinder touches the bottom of the tank, forming a dose or slug of material inside the hollow cylinder; a downward movement by the piston to compress the dose of pharmaceutical material; a subsequent return upward movement by the hollow cylinder with the compressed dose still inside it, to pick up the dose from the tank; finally, with a downward thrust movement by the piston, the compressed dose is released into the relative capsule body, after an axial movement designed to align the capsule body on the raised hollow cylinder, by rotation of the drum.
To correctly pick up the dose then discharge it into the capsule body but, above all, to ensure that each capsule contains a dose of pharmaceutical material whose weight lies within a predetermined weight range, the vertical stroke of the piston operating inside the hollow cylinder is suitably regulated, in both directions, according to values defined as constants and synchronised with the movement of the hollow cylinder.
To check that the weight of the doses of pharmaceutical material in the capsules is correct, one known check method involves the use of precision scales on which capsules taken as samples from a capsule filling machine outfeed portion are placed.
Since such scales have high settling times during weighing steps, this method cannot be used to check all of the capsules produced by the capsule filling machine, as this would greatly slow down the production flow.
To solve the above-mentioned problem, that is to say, to check all of the capsules produced without slowing down the production flow, at present modern capsule filling machines are fitted with check devices designed to detect the downward force of the pistons during the dosing step and to control the piston stroke with feedback if said force gives doses of pharmaceutical material with unacceptable weight values.
In the capsule filling machine described, for example in U.S. Pat. No. 6,327,835, each dosing piston of a drum rotating with alternating motion is fitted with a force sensor, connected by connecting cable transmission systems to a control unit designed to receive, during each pause in the drum alternating motion, a signal relative to a piston compression force value, to compare said value with a predetermined reference value and to send a feedback signal to adjust the piston drive unit during the pharmaceutical material dosing steps.
The control device described in said U.S. patent is validly used only on capsule filling machines with alternating motion but, due to the connecting cable transmission systems, can obviously not be used on a capsule filling machine whose drum rotates continuously at a speed of rotation which can currently be very high.